Network availability and resilience are important values of SD-WANs. By using multiple low cost internet circuits, with the addition of LTE, you can reach a network availability that theoretically will never fail, rather cost effectively. While your particular requirements might still require a primary MPLS path, the same mathematics applies.
Your various WAN circuits will be operating in parallel, so a complete failure will require all to fail. It used to be that your local loop point of entry was your weakest link, since multiple circuits could fail from a “back-hoe” incident or physical break. Adding LTE or Cellular Access eliminates this risk, enhancing your availability. Remember that all carriers will have maintenance windows that impact availability. If your network operations might be impacted by “off-hours” maintenance windows, then multiple circuits will protect you from this disruption.
How to Calculate Network Availability
The combined network availability is calculated by the equation: Site Availability = 1-((1-Service A Availability)*(1-Service B Availability)*(1-Service N Availability)) which can also be written as:
A = 1-(1-Ax )n if all the circuits have the same availability.
Where n is the number of circuits in parallel and Ax the availability of each circuit.
The implications of the above equation are that the combined availability of multiple circuit in parallel is always much higher than the availability of its individual circuit. The table below shows the network availability and downtime for individual consumer grade 99% availability circuits and the parallel combinations.
| Component | Availability | Downtime |
| X | 99.0% (2-nines) | 5256 min/year (7.3 hours/mo) |
| Two X circuits operating in parallel | 99.99% (4-nines) | 52.6 min/year (4.4 minutes/mo) |
| Three X circuits operating in parallel | 99.9999% (6-nines) | 0.526 min/year (2.6 seconds/mo) |
| Four X circuits operating in parallel | 99.999999% (8-nines) | 0.00526 min/year (0.026 seconds/mo) |
LTE can save the day
Obviously, if all the circuits are entering your building at the same point or carried in the same aerial or conduit path, this single point of failure makes these calculations useless, in reality. Which is where LTE saves the day! With Broadband and LTE, you can assure that your enterprise has a fault tolerant network, at a very reasonable cost. Using one circuits with a 99.9% availability along with a 99% availability Broadband circuit and LTE, you will have a theoretical availability of 7 nines (99.99999)
Thus parallel circuits provides a very powerful mechanism for making a highly reliable network from lower reliability circuits.
